In general, the resistivity of a grown or diffused layer in a semiconductor, the resistivity of a metallic film produced by vapor deposition, or the resistivity of an electrically conductive film, is one important parameter which represents the electrical characteristics of these materials. In particular, surface resistivity, which is resistance per unit surface area, is often used as a measure of the conductive level of an electrically conductive polymeric material employed as a raw material for a conductive film. Here the surface resistivity is specified by the electrical resistance value between opposing sides of a unit square.
With regard to such measurement of surface resistivity of a conductive film, the applicant has previously filed a patent application (Japanese Utility Model application No. 60-71797, filed on May 14, 1985) on a simple surface resistivity measurement device capable of measuring the surface resistivity of a sample non-destructively in a simple and inexpensive manner.
In the simple surface resistivity measurement device according to this previously filed application, a holder is constructed of an electrical insulating material, and four spring-contact probes spaced apart a fixed distance are secured on the top surface of the holder. Contactors at the lower ends of these probes are urged by the elastic force of the springs so as to project slightly below the lower surface of both legs of the holder.
Since the simple surface resistivity measuring device is so constructed that the four spring-contact probes are thus provided on the holder consisting of an electrical insulator, the probes are brought into abutting contact with the sample at a fixed angle and pressure at all times by the elastic force of the springs, so that the surface resistivity of the sample can be measured accurately and non-destructively.
In general, when measuring surface resistivity, the contact resistance of electrodes contacting the sample or the resistance of the lead wires begins to appear with a two-terminal method if the surface resistivity measured is less than 10.sup.2 .OMEGA./.quadrature.. When the surface resistivity is less than 10.sup.0 .OMEGA./.quadrature., accurate measurement cannot be expected unless the four-point probe method is employed.
FIG. 2 is a view illustrating a method of measuring surface resistivity by a two-point probe method according to the prior art. A current I is passed through both ends of a sample 25. Letting V represent the potential difference across the ends of the sample at this time, it will be understood that the resistance value R of the sample is given by V/I. Let the surface area of the sample 25 be represented by S (cm.sup.2), and let the length be l (cm). The volume resistivity .rho..sub.v (.OMEGA..multidot.cm) will then be given by .rho..sub.v =R.times.S/l (.OMEGA..multidot.cm). If the sample 25 is an isotropic conductor, then the surface resistivity .rho..sub.S (.OMEGA./.quadrature.) and the volume resistivity .rho..sub.v (.OMEGA..multidot.cm) will be related as follows: EQU .rho..sub.S =.rho..sub.v /t=R.multidot.w/l (.OMEGA./.quadrature.)
where t (cm), w and l represents the thickness, width and length of the sample, respectively.
It should be noted that although the unit of surface resistivity is the ohm, this is expressed here as .OMEGA./.quadrature. in terms of surface resistivity per unit square in order to distinguish it from a resistance value between any two points.
With the above-described two-terminal method, the contact resistance between the sample and the probes contributes to the electrical potential across the probes, and for this reason the surface resistivity of a sample having a comparatively low resistivity cannot be accurately determined. A measurement method for eliminating the contact resistance between a sample and a probe is the four-point probe method, which is now the method most widely used. According to this method, four probes arranged in a straight line are brought into contact with a sample, a current I is passed through the two outer probes, and the potential V across the two inner probes is measured. The volume resistivity .rho..sub.V of the sample is given by .rho..sub.V =F.multidot.t.multidot.V/I, where t is the thickness of the sample. Here the coefficient F is dependent upon the shape and dimensions of the sample and the position of the probe array. Conventionally, this coefficient is decided based on the assumption that the sample has infinite area in comparison with the probe spacing, and it does not take the thickness of the sample into consideration. Consequently, large errors appear in the measured resistivity values of the sample depending upon the thickness, shape and size of the sample and the measurement positions.
In the method of measuring surface resistivity using the simple surface resistivity measuring device, the spring contact probes contact the sample at different contact pressures if the probes are brought into abutting contact with the sample in a state where the holder is tilted with respect to the sample. When the contact pressures differ, the area over which the tip of each contacts the sample differs from one probe to the next. As a result, if the angle of inclination of the holder differs when the operation for measuring the same sample is repeated, the measured value will differ each time.
Further, in the prior art, the sample to be measured is set while exposed to the atmosphere. This makes it very difficult to keep the sample at a predetermined temperature. As a consequence, it is almost impossible to measure the temperature dependence of the sample surface resistivity.
A method of measuring surface resistivity by the four-point probe method is illustrated in FIG. 20. The four-point probe is constructed by arranging four probes (electrodes) 40 through 43 equidistantly in a straight line. The probe array is placed on a sample, the voltage V produced across the two inner probes when a constant current is passed through the two outer probes is measured, and the surface resistivity is determined using a mathematical formula based on the voltage and current values, etc.
With this four-point probe, however, the spacing between adjacent probes cannot be made very small due to manufacturing limitations when the probe is fabricated. More specifically, owing to limitations imposed by a manual soldering operation for connecting the probes to their lead wires, it is difficult to make the spacing between electrodes (probes) less than about 1.5 mm. Accordingly, when the conventional probe arrangement is used, the distance between the two outer probes is about 5 mm, so that the entire four-point probe array cannot be placed on a sample whose length is less than 5 mm. This makes it impossible to measure the surface resistivity of such a sample.